Engineering and Construction

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Deep foundations are one of the most effective means of mitigating foundation movement during an earthquake. For example, the Niigata Earthquake resulted in dramatic damage due to liquefaction of the sand deposits in the low-lying areas of Niigata City. At the time of the Niigata Earthquake, there were approximately 1500 reinforced concrete buildings in Niigata City and about 310 of these buildings were damaged, of which approximately 200 settled or tilted rigidly without appreciable damage to the superstructure. The damaged concrete buildings were built on very shallow foundations or friction piles in loose soil. Similar concrete buildings founded on piles bearing on firm strata at a depth of 66 ft (20 m) did not suffer damage.

There are several important earthquake design considerations when using deep foundations, such as piles or piers, as follows:

1. Connection between pile and cap. It is important to have an adequate connection between the top of the pile and the pile cap. This can be accomplished by using steel reinforcement to connect the pile to the pile cap. Without this reinforced connection, the pile will be susceptible to separation at the pile cap during the earthquake.

2. Downdrag loads due to soil liquefaction. The pile-supported structure may remain relatively stationary, but the ground around the piles may settle as the pore water pressures dissipate in the liquefied soil. The settlement of the ground relative to the pile will induce downdrag loads onto the pile. The piles should have an adequate capacity to resist the downdrag loads.

The relative movement between the relatively stationary structure and the settling soil can also damage utilities. To mitigate damage to utilities, flexible connections can be provided at the location where the utilities enter the building.

3. Passive resistance for liquefiable soil. A common assumption is that the liquefied soil will be unable to provide any lateral resistance. If a level-ground site contains an upper layer of nonliquefiable soil that is of sufficient thickness to prevent ground fissuring and sand boils, then this layer may provide passive resistance for the piles, caps, and grade beams.

4. Liquefaction of sloping ground. For liquefaction of sloping ground, there will often be lateral spreading of the ground, which could shear-off the piles. One mitigation measure consists of the installation of compaction piles, in order to create a zone of nonliquefiable soil around and beneath the foundation.

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In southern California, the most common types of shallow foundations used for single-family houses and other lightly loaded structures are a raised wood floor foundation and a concrete slab-on-grade, as follows:

Raised Wood Floor Foundation. The typical raised wood floor foundation consists of continuous concrete perimeter footings and interior (isolated) concrete pads. The floor beams span between the continuous perimeter footings and the isolated interior pads. The continuous concrete perimeter footings are typically constructed so that they protrude about 1 to 2 ft (0.3 to 0.6 m) above adjacent pad grade. The interior concrete pad footings are not as high as the perimeter footings, and short wood posts are used to support the floor beams. The perimeter footings and interior posts elevate the wood floor and provide for a crawl space below the floor. In general, damage caused by southern California earthquakes has been more severe to houses having a raised wood floor foundation. There may be several different reasons for this behavior:

1. Lack of shear resistance of wood posts. As previously mentioned, in the interior, the raised wood floor beams are supported by short wood posts bearing on interior concrete pads. During the earthquake, these short posts are vulnerable to collapse or tilting during the earthquake.

2. No bolts or inadequate bolted condition. Because in many cases the house is not adequately bolted to the foundation, it can slide or even fall off of the foundation during the earthquake. In other cases the bolts are spaced too far apart and the wood sill plate splits, allowing the house to slide off the foundation.

3. Age of residence. The houses having this type of raised wood floor foundation are often older.
The wood is more brittle and in some cases weakened due to rot or termite damage. In some cases, the concrete perimeter footings are nonreinforced or have been weakened due to prior soil movement, making them more susceptible to cracking during the earthquake.

4. Crawl space vents. In order to provide ventilation to the crawl space, long vents are often constructed just above the concrete foundation. Such vents provide areas of weakness just above the foundation.

All of these factors can contribute to the detachment of the house from the foundation. Besides determining the type of foundation to resist earthquake related effects, the foundation engineer could also be involved with the retrofitting of existing structures. For example, to prevent the house from sliding off of the foundation, bolts or tie-down anchors could be installed to securely attach the wood framing to the concrete foundation. Wood bracing or plywood could also be added to the open areas between posts to give the foundation more shear resistance.

Slab-on-Grade Foundation. In southern California, the concrete slab-on-grade is the most common type of foundation that is presently used for houses and other lightly loaded structures. Typical slab-on-grade foundations consist of either the conventional or the posttensioned foundation.

The conventional slab-on-grade foundation consists of perimeter and interior continuous footings, interconnected by the slab-on-grade. Construction of the conventional slab-on-grade begins with the excavation of the interior and perimeter continuous footings. Steel reinforcing bars are commonly centered in the footing excavations and wire mesh or steel bars are used as reinforcement for

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The usual approach for settlement analyses is to first estimate the amount of earthquake-induced total settlement (ρmax) of the foundation. Because of variable soil conditions and structural loads, the earthquake-induced settlement  is rarely uniform. A common assumption is that the maximum differential settlement Δ of the foundation will be equal to 50 to 75 percent of ρmax (i.e., 0.5 ρmax ≤ Δ ≤ 0.75 ρmax ). If the anticipated total settlement ρmax and/or the maximum differential settlement are deemed to be unacceptable, then remedial measures are needed. One alternative is soil improvement, which will be discussed later.

Instead of soil improvement, the foundation can be designed to resist the anticipated soil movement caused by the earthquake. For example, mat foundations or posttensioned slabs may enable the building to remain intact, even with substantial movements. Another option is a deep foundation system that transfers the structural loads to adequate bearing material in order to bypass a compressible or liquefiable soil layer. A third option is to construct a floating foundation, which is a special type of deep foundation where the weight of the structure is balanced by the removal of soil and construction of an underground basement. A floating foundation could help reduce the amount of rocking settlement caused by the earthquake.

1 Shallow Foundations
2 Deep Foundations

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In order for the active wedge to be developed, there must be sufficient movement of the retaining wall. There are many cases where movement of the retaining wall is restricted. Examples include massive bridge abutments, rigid basement walls, and retaining walls that are anchored in non-yielding rock. These cases are often described as restrained retaining walls.

For earthquake conditions, restrained retaining walls will usually be subjected to larger forces as compared to those retaining walls that have the ability to develop the active wedge. Provided the soil is not weakened during the earthquake, one approach is to use the pseudostatic method in order to calculate the earthquake force, with an increase to compensate for the unyielding wall conditions, or:

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The pseudostatic method has been previously discussed and it is applicable for slope stability and retaining wall analyses. The advantages of this method are that it is easy to understand and apply. Similar to earthquake slope stability analyses, this method ignores the cyclic nature of the earthquake and treats it as if it applies an additional static force upon the retaining wall. In particular, the pseudostatic approach is to apply a lateral earthquake force upon the retaining wall. To derive the lateral force, it can be assumed that the force acts through the centroid of the active wedge. The pseudostatic lateral force PE is calculated by using Eq. 13.12 (i.e., PE = kh W) and it has units of pounds per linear foot of wall length or kilonewtons per linear meter of wall length.

Note that an earthquake could subject the active wedge to both vertical and horizontal pseudostatic forces. However, the vertical force is usually ignored in the standard pseudostatic analysis.

This is because the vertical pseudostatic force acting on the active wedge usually has much less effect on the design of the retaining wall. In addition, most earthquakes produce a peak vertical acceleration that is less than the peak horizontal acceleration, and hence kv is smaller than kh.

The only unknowns in the pseudostatic method are the weight of the active wedge (W) and the seismic coefficient kh. Because of the usual relatively small size of the active wedge, the seismic coefficient kh can be assumed to be equal to amax /g. Using Fig. 11.3, the weight of the active wedge can be calculated as follows:



Using Eq. 14.1, the final result is as follows:


Note that since the pseudostatic force is applied to the centroid of the active wedge, the location of the force PE is at a distance of 2/3H above the base of the retaining wall. Seed and Whitman (1970) developed a similar equation that can be used to determine the horizontal pseudostatic force acting on the retaining wall, as follows:


Note that the terms in Eq 14.3 have the same definitions as the terms in Eq. 14.2. In comparing
Eqs. 14.2 and 14.3, the two equations are identical for the case where

According to
Seed and Whitman (1970), the location of the pseudostatic force from Eq. 14.3 can be assumed to act at a distance of 0.6 H above the base of the wall.

Mononobe and Matsu (1929) and Okabe (1926) also developed an equation that can be used to determine the horizontal pseudostatic force acting on the retaining wall. This method is often referred to as the Mononobe-Okabe method. The equation is an extension of the Coulomb approach and is as follows:

where PAE is the sum of the static (PA) and the pseudostatic earthquake force (PE). The equation for kAE is shown in Fig. 11.4. Note that in Fig. 11.4, the term y is defined as follows:

 The original approach by Mononobe and Okabe was to assume that the force PAE from Eq. 14.4 acts at a distance of 1/3 H above the base of the wall.

For the analysis of sliding and overturning of the retaining wall, it is common to accept a lower factor of safety (1.1 to 1.2) under the combined static and earthquake loads. Thus the retaining wall would be considered marginally stable for the earthquake sliding and overturning conditions. Note in the above table that the factor of safety for overturning is equal to 2.35 based on the Mononobe-Okabe method. This factor of safety is much larger than the other two methods. This is because the force PAE is assumed to be located at a distance of 1/3 H above the base of the wall. Kramer (1996) suggests that it is more appropriate to assume that PE is located at a distance of 0.6 H above the base of the wall (i.e., PE = PAE – PA, see Eq. 14.4).

Although the calculations are not shown, it can be demonstrated that the resultant location of N for the earthquake condition is outside the middle third of the footing. Depending on the type of material beneath the footing, this condition could cause a bearing capacity failure or excess settle- ment at the toe of the footing during the earthquake.

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The purpose of this section is to describe retaining wall design considerations for earthquakes. The performance of retaining walls during earthquakes is very complex. As stated by Kramer (1996), laboratory tests and analyses of gravity walls subjected to seismic forces have indicated the following:

1. Walls can move by translation and/or rotation. The relative amounts of translation and rotation depend on the design of the wall; one or the other may predominate for some walls, and both may occur for others (Nadim and Whitman, 1984; Siddharthan et al., 1992).

2. The magnitude and distribution of dynamic wall pressures are influenced by the mode of wall movement, e.g., translation, rotation about the base, or rotation about the top (Sherif et al., 1982; Sherif and Fang, 1984a,b).

3. The maximum soil thrust acting on a wall generally occurs when the wall has translated or rotated toward the backfill (i.e., when the inertial force on the wall is directed toward the backfill). The minimum soil thrust occurs when the wall has translated or rotated away from the backfill.

4. The shape of the earthquake pressure distribution on the back of the wall changes as the wall moves. The point of application of the soil thrust therefore moves up and down along the back of the wall. The position of the soil thrust is highest when the wall has moved toward the soil and lowers when the wall moves outward.

5. Dynamic wall pressures are influenced by the dynamic response of the wall and backfill and can increase significantly near the natural frequency of the wall-backfill system (Steedman and Zeng, 1990). Permanent wall displacements also increase at frequencies near the natural frequency of the wall-backfill system (Nadim, 1982). Dynamic response effects can also cause deflections of different parts of the wall to be out of phase.

This effect can be particularly significant for walls that penetrate into the foundation soils when the backfill soils move out of phase with the foundation soils.

6. Increase residual pressures may remain on the wall after an episode of strong shaking has ended
(Whitman, 1990).

Because of the complex soil-structure interaction during the earthquake, the most commonly used method for the design of retaining walls is the pseudostatic method.

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Tsunami is a Japanese word that when translated into English means harbor wave and it is defined as an ocean wave that is created by a disturbance that vertically displaces a column of seawater.Tsunami can be generated by an oceanic meteorite impact, submarine landslide, or earthquake if the sea floor abruptly deforms and vertically displaces the overlying water. Earthquakes generated at sea-floor subduction zones are particularly effective in generating tsunamis. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium.

Tsunami is different from a normal ocean wave in that it has a long period and wavelength. For example, tsunami can have a wavelength in excess of 60 mi (100 km) and a period on the order of 1 h.

In the Pacific Ocean, where the typical water depth is about 13,000 ft (4000 m), tsunami travels at about 650 ft/sec (200 m/sec). Because the rate at which a wave loses its energy is inversely related to its wavelength, tsunami not only propagates at high speeds, it also can travel long transoceanic distances with limited energy losses.

The tsunami is transformed as it leaves the deep water of the ocean and travels into the shallower water near the coast. The tsunami’s speed diminishes as it travels into the shallower coastal water and its height grows. While the tsunami may be imperceptible at sea, the shoaling effect near the coast causes the tsunami to grow to be several meters or more in height. When it finally reaches the coast, the tsunami may develop into a rapidly rising or falling tide, a series of breaking waves, or a tidal bore.

Seiche is identical to tsunami, except that it occurs in an inland body of water, such as a lake. It can be caused by lake-bottom earthquake movements or by volcanic eruptions and landslides with- in the lake. Seiche has been described as being similar to the sloshing of water in a bathtub.

Because of the tremendous destructive forces, options to mitigate structural damage caused by tsunami or seiche are often limited. Some possibilities include the construction of walls to deflect the surging water or the use of buildings having weak lower-floor partitions that will allow the water to flow through the building, rather than knocking it down.

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Pounding damage can occur when two buildings are constructed close to each other and as they rock back-and-forth during the earthquake, they collide into each other. Even when two buildings having dissimilar construction materials or different heights are constructed adjacent to each other, it does not necessarily mean that they will be subjected to pounding damage.

The common situation for pounding damage is when a much taller building, which has a higher period and larger amplitude of vibration, is constructed against a squat and short building that has a lower period and smaller amplitude of vibration. Thus during the earthquake, the buildings will vibrate at different frequencies and amplitudes, and they can collide into each other. The effects of pounding can be especially severe if the floors of one building impact the other building at different elevations, so that, for example, the floor of one building hits a supporting column of an adjacent building.

It is very difficult to model the pounding effects of two structures and hence design structures to resist such damage. As a practical matter, the best design approach to prevent pounding damage is to provide sufficient space between the structures in order to avoid the problem. If two buildings must be constructed adjacent to each other, then one design feature should be to have the floors of both buildings at the same elevations, so that the floor of one building does not hit a supporting column of an adjacent building.

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It is generally recognized that single-family wood frame structures that include shear walls in their construction are very resistant to collapse from earthquake shaking. This is due to several factors, such as their flexibility, strength, and light dead loads, which produce low earthquake-induced inertia loads. These factors make the wood-frame construction much better at resisting shear forces and hence more resistant to collapse.

There are exceptions to the general rule that wood frame structures are resistant to collapse. For example, in the 1995 Kobe earthquake, the vast majority of deaths were due to the collapse of one- and two-story residential and commercial wood-frame structures. More than 200,000 houses, about 10 percent of all houses in the Hyogo prefecture, were damaged, including over 80,000 collapsed houses, 70,000 severely damaged, and 7000 consumed by fire. The collapse of the houses has been attributed to several factors, such as (EQE Summary Report, 1995):

• Age-related deterioration, such as wood rot, that weakened structural members.
• Post and beam construction that often included open first floor areas (i.e., a soft first floor), with few interior partitions that were able to resist lateral earthquake loads.
• Weak connections between the walls and the foundation.
• Inadequate foundations that often consisted of stones or concrete blocks.
• Poor soil conditions consisting of thick deposits of soft or liquefiable soil that settled during the earthquake. Because of the inadequate foundations, the wood-frame structures were unable to accommodate the settlement.
• Inertia loads from heavy roofs that exceeded the lateral earthquake load-resisting capacity of the supporting walls. The heavy roofs were created by using thick mud or heavy tile and were used to resist the winds from typhoons. However, when the heavy roofs collapsed during the earthquake, they crushed the underlying structure.

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There are many different types of structural systems that can be used to resist the inertia forces in a building that are induced by the earthquake ground motion. For example, the structural engineer could use braced frames, moment resisting frames, and shear walls to resist the lateral earthquake- induced forces. Shear walls are designed to hold adjacent columns or vertical support members in place and then transfer the lateral forces to the foundation. The forces resisted by shear walls are predominantly shear forces, although a slender shear wall could also be subjected to significant bending (Arnold and Reitherman, 1982).

Common problems with shear walls are that they have inadequate strength to resist the lateral forces and that they are inadequately attached to the foundation. For example, having inadequate shear walls on a particular building level can create a soft story. A soft story could also be created if there is a discontinuity in the shear walls from one floor to the other, such as a floor where its shear walls are not aligned with the shear walls on the upper or lower floors.

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Pancaking occurs when the earthquake shaking causes a soft story to collapse, leading to total failure of the overlying floors. These floors crush and compress together such that the final collapsed condition of the building consists of one floor stacked on top of another, much like a stack of pancakes.

Pancaking of reinforced concrete multistory buildings was common throughout the earthquake-stricken region of Turkey due to the Kocaeli earthquake on August 17, 1999. Concerning the damage caused by the Kocaeli earthquake, Bruneau (1999) states:

Pancaking is attributed to the presence of ‘soft’ lower stories and insufficiently reinforced connections at the column-beam joints. Most of these buildings had a ‘soft’ story—a story with most of its space unen-closed—and a shallow foundation and offered little or no lateral resistance to ground shaking. As many as 115,000 of these buildings—some engineered, some not—were unable to withstand the strong ground shaking and were either badly damaged or collapsed outright, entombing sleeping occupants beneath the rubble. Partial collapses involved the first two stories. The sobering fact is that Turkey still has an existing inventory of several hundred thousand of these highly vulnerable buildings. Some will need to under- go major seismic retrofits; others will be demolished.

An example of pancaking caused by the Kocaeli earthquake is shown in Fig. 14.6.

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A soft story, also known as a weak story, is defined as a story in a building that has substantially less resistance, or stiffness, than the stories above or below it. In essence, a soft story has inadequate shear resistance or inadequate ductility (energy absorption capacity) to resist the earthquake-induced building stresses. Although not always the case, the usual location of the soft story is at the ground floor of the building. This is because many buildings are designed to have an open first-floor area that is easily accessible to the public. Thus the first floor may contain large open areas between columns, without adequate shear resistance. The earthquake-induced building movement also causes the first floor to be subjected to the greatest stress, which compounds the problem of a soft story on the ground floor.

Concerning soft stories, the National Information Service for Earthquake Engineering (2000) states:

In shaking a building, an earthquake ground motion will search for every structural weakness. These
weaknesses are usually created by sharp changes in stiffness, strength and/or ductility, and the effects of these weaknesses are accentuated by poor distribution of reactive masses. Severe structural damage suffered by several modern buildings during recent earthquakes illustrates the importance of avoiding sudden changes in lateral stiffness and strength. A typical example of the detrimental effects that these discontinuities can induce is in the case of buildings with a ‘soft story.’ Inspections of earthquake damage as well as the results of analytical studies have shown that structural systems with a soft story can lead to serious problems during severe earthquake ground shaking. [Numerous examples] illustrate such damage and therefore emphasize the need for avoiding the soft story by using an even distribution of flexibility, strength, and mass.

Three examples of buildings having a soft story on the ground floor are as follows:

1. Chi-chi earthquake in Taiwan on September 21, 1999. In Taiwan, it is common practice to have an open first floor area by using columns to support the upper floors. In some cases, the spaces between the columns are filled-in with plate-glass windows in order to create ground-floor shops.

Figure 14.1 shows an example of this type of construction and the resulting damage caused by the Chi-chi earthquake.




2. Northridge earthquake in California on January 17, 1994. Many apartment buildings in southern California contain a parking garage on the ground floor. In order to provide an open area for the ground floor parking area, isolated columns are used to support the upper floors. These isolated columns often do not have adequate shear resistance and are susceptible to collapse during an earthquake. For example, Figs. 14.2 and 14.3 show the collapse of an apartment building during the Northridge earthquake caused by the weak shear resistance of the first floor garage area.






3. Kocaeli earthquake in Turkey on August 17, 1999. In terms of building conditions during this earthquake, it has been stated (Bruneau, 1999):

A typical reinforced concrete frame building in Turkey consists of a regular, symmetric floor plan, with square or rectangular columns and connecting beams. The exterior enclosures as well as interior partitioning are of non-bearing unreinforced brick masonry infill walls. These walls contributed significantly to the lateral stiffness of buildings during the earthquake and, in many instances, controlled the lateral drift and resisted seismic forces elastically. This was especially true in low-rise buildings, older buildings where the ratio of wall to floor area was very high, and buildings located on firm soil. Once the brick infills failed, the lateral strength and stiffness had to be provided by the frames alone, which then experienced significant inelasticity in the critical regions. At this stage, the ability of reinforced concrete columns, beams, and beam-column joints to sustain deformation demands depended on how well the seismic design and detailing requirements were followed both in design and in construction.

A large number of residential and commercial buildings were built with soft stories at the first-floor level. First stories are often used as stores and commercial areas, especially in the central part of cities.
 
These areas are enclosed with glass windows, and sometimes with a single masonry infill at the back.

Heavy masonry infills start immediately above the commercial floor. During the earthquake, the presence of a soft story increased deformation demands very significantly, and put the burden of energy dissipation on the first-story columns. Many failures and collapses can be attributed to the increased deformation demands caused by soft stories, coupled with lack of deformability of poorly designed columns. This was particularly evident on a commercial street where nearly all buildings collapsed towards the street.

Examples of this soft story condition are shown in Figs. 14.4 and 14.5.





Concerning the retrofitting of a structure that has a soft story, the National Information Service for Earthquake Engineering (2000) states:

There are many existing buildings in regions of high seismic risk that, because of their structural systems and/or of the interaction with non-structural components, have soft stories with either inadequate shear resistance or inadequate ductility (energy absorption capacity) in the event of being subjected to severe earthquake ground shaking. Hence they need to be retrofitted. Usually the most economical way of retrofitting such a building is by adding proper shear walls or bracing to the soft stories.

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The purpose of this section is to present examples of structural damage caused by earthquakes. A lot of
this damage is related to foundation conditions.

1 Torsion   Torsional problems develop when the center of mass of the structure is not located at the center of its lateral resistance, which is also known as the center of rigidity. A common example is a tall building that has a first floor area consisting of a space that is open and supports the upper floors by the use of isolated columns, while the remainder of the first floor area contains solid load-bearing walls that are interconnected.

The open area having isolated columns will typically have much less lateral resistance than that part of the floor containing the interconnected load-bearing walls. While the center of mass of the building may be located at the mid-point of the first floor area, the center of rigidity is offset toward the area containing the interconnected load-bearing walls. During the earthquake, the center of mass will twist about the center of rigidity, causing torsional forces to be induced into the building frame.

2 Soft Story   A soft story, also known as a weak story, is defined as a story in a building that has substantially less resistance, or stiffness, than the stories above or below it. In essence, a...

3 Pancaking   Pancaking occurs when the earthquake shaking causes a soft story to collapse, leading to total failure of the overlying floors. These floors crush and compress together such that...

4 Shear Walls   There are many different types of structural systems that can be used to resist the inertia forces in a building that are induced by the earthquake ground motion. For...

5 Wood-Frame Structures   It is generally recognized that single-family wood frame structures that include shear walls in their construction are very resistant to collapse from earthquake shaking. This...

6 Pounding Damage  Pounding damage can occur when two buildings are constructed close to each other and as they rock back-and-forth during the earthquake, they collide into each other...

7 Tsunami and Seiche   Tsunami is a Japanese word that when translated into English means harbor wave and it is defined as an ocean wave that is created by a disturbance that vertically displaces...

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INTRODUCTION
As discussed previously, the actual rupture of the ground due to fault movement could damage a structure. Secondary effects, such as the liquefaction of loose granular soil and slope movement or failure could also cause structural damage. This chapter will discuss some of the other earthquake-induced effects or structural conditions that can result in damage to foundations and retaining walls.

Topics will include earthquake-induced settlement and foundation alternatives to mitigate earth-quake effects.

Earthquakes throughout the world cause a considerable amount of death and destruction.

Earthquake damage can be classified as being either structural or nonstructural. For example, the Federal Emergency Management Agency (1994) states:

Damage to buildings is commonly classified as either structural or non-structural. Structural damage means the building’s structural support has been impaired. Structural support includes any vertical and lateral force resisting systems, such as the building frames, walls, and columns. Non-structural damage does not affect the integrity of the structural support system. Examples of non-structural damage include broken windows, collapsed or rotated chimneys, and fallen ceilings. During an earthquake, buildings get thrown from side to side, and up and down. Heavier buildings are subjected to higher forces than light-weight buildings, given the same acceleration. Damage occurs when structural members are overloaded, or differential movements between different parts of the structure strain the structural components. Larger earthquakes and longer shaking durations tend to damage structures more. The level of damage resulting from a major earthquake can be predicted only in general terms, since no two buildings undergo the exact same motions during a seismic event. Past earthquakes have shown us, however, that some buildings are likely to perform more poorly than others.

There are four main factors that cause structural damage during an earthquake, as follows:

1. Strength of shaking. For small earthquakes (magnitude less than 6), the strength of shaking decreases rapidly with distance from the epicenter of the earthquake. According to the United States Geological Survey (2000b), the strong shaking along the fault segment that slips during a earthquake becomes about half as strong at a distance of 8 mi, a quarter as strong at a distance of 17 mi, an eighth as strong at a distance of 30 mi, and a sixteenth as strong at a distance of 50 mi.

In the case of a small earthquake, the center of energy release and the point where slip begins is not far apart. But in the case of large earthquakes, which have a significant length of fault rup- ture, these two points may be hundreds of miles apart. Thus for big earthquakes, the strength of shaking decreases in a direction away from the fault rupture.

2. Length of shaking. The length of shaking depends on how the fault breaks during the earthquake. For example, the maximum shaking during the Loma Prieta earthquake lasted only 10 to 15 sec But during other magnitude earthquakes in the San Francisco bay area, the shaking may last 30 to 40 sec. The longer the ground shakes, the greater the potential for structural damage. In general, the higher the magnitude of an earthquake,  the longer the duration of the shaking ground (see Table 13.2).



3. Type of subsurface conditions. Ground shaking can be increased if the site has a thick deposit of soil that is soft and submerged. Many other subsurface conditions can cause or contribute to structural damage. For example, as discussed in Sec. 13.4, there could be structural damage due to liquefaction of loose submerged sands.

4. Type of building. Certain types of buildings and other structures are especially susceptible to the side-to-side shaking common during earthquakes. For example, sites located within approxi- mately 10 mi (16 km) of the epicenter or location of fault rupture are generally subjected to rough, jerky, and high frequency seismic waves that are often more capable of causing short buildings to vibrate vigorously. For sites located at greater distance, the seismic waves often develop into longer period waves that are more capable of causing high-rise buildings and buildings with large floor areas to vibrate vigorously (Federal Emergency Management Agency, 1994).

Much like diseases will attack the weak and infirm, earthquakes damage those structures that have inherent weaknesses or age-related deterioration. Those buildings that are nonreinforced, poorly constructed, weakened from age or rot, or underlain by soft or unstable soil are most susceptible to damage. The next section will discuss some of these susceptible structures.

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INTRODUCTION
Geotechnical earthquake engineering can be defined as that subspecialty within the field of geotechnical engineering that deals with the design and construction of projects in order to resist the effects of earthquakes. Geotechnical earthquake engineering requires an understanding of basic geotechnical principles as well as an understanding of geology, seismology, and earthquake engineering. In a broad sense,  seismology can be defined as the study of earthquakes. This would include the internal behavior of the earth and the nature of seismic waves generated by the earthquake.

For geotechnical earthquake engineering of soils and foundations, the types of activities that may need to be performed by the geotechnical engineer include the following:

• Subsurface exploration for geotechnical earthquake engineering, i.e., the screening investigation and the quantitative evaluation.
• Total stress and effective stress analyses for geotechnical earthquake engineering
• Bearing capacity analyses for geotechnical earthquake engineering
• For the design earthquake, determining the peak ground acceleration and magnitude of the earthquake
• Investigating the possibility of liquefaction at the site
• Investigating the stability of slopes for the additional forces imposed during the design earthquake
• Calculating the settlement of the foundation caused by the design earthquake
• Performing calculations to assess the effect of the earthquake on retaining walls
• Considering foundation alternatives to mitigate the effects of earthquakes

In terms of the accuracy of the calculations used to determine the earthquake induced soil move-
ment, Tokimatsu and Seed (1984) conclude:

It should be recognized that, even under static loading conditions, the error associated with the estimation of settlement is on the order of ±25 to 50%. It is therefore reasonable to expect less accuracy in predicting settlements for the more complicated conditions associated with earthquake loading.... In the application of the methods, it is essential to check that the final results are reasonable in the light of available experience.